1. Field of the Invention
This invention relates to the in-situ combustion of oil shale to produce shale oil, and more particularly to a method of disposing of spent oil shale and stabilizing in-situ retorts in oil shale formations.
2. Description of the Prior Art
The oil shale deposits in the western states of the United States extend over thousands of square miles and over large areas are more than 1000 feet in thickness. Oil shale contains kerogen, a solid carbonaceous material which on heating to a temperature above about 800.degree. F. yields shale oil. The oil shale deposits may produce from about 15 to 80 gallons of shale oil per ton of shale.
One method of recovering the oil from oil shale is to treat the oil shale at high temperatures in retorts located at the ground surface. Few of the shale deposits are located near enough to the ground surface to permit strip mining; consequently, expensive underground mining methods are necessary for removing oil shale from the deposits and delivering it to the surface. U.S. Pat. No. 3,588,175 of Whiting describes a number of different mining techniques for the mining of oil shale to be retorted at the surface. The cost of underground mining operations and lifting to the surface for retorting the volume of oil shale that would be necessary to produce shale oil has been a major factor in delaying production of shale oil. Moreover, disposal of spent shale produced in surface retorting is a difficult problem. U.S. Pat. Nos. 3,588,175 of Whiting and 3,459,003 of O'Neal suggest filling the mined-out cavity formed in his mining operation with an aqueous slurry of spent shale from surface retorting. The slurry contains excess water which separates from the spent shale and is removed from the mined-out cavity either by pumping or draining. Whiting states that the compaction of the fill as the height of fill increases causes movement against the side and thereby reduces the load on the bulkheads at the bottom of the mined-out cavity. O'Neal increases the strength of the mass by manufacturing a cement from a portion of the surface-retorted shale and pumping a slurry of the cement into the cavity.
Because of the expense of underground mining of shale, it has been proposed that the shale be retorted in-situ. The low permeability of shale makes it necessary to rubblize a retort in the shale deposit before the flow of air and combustion products necessary for in-situ combustion can be maintained. Rubblization is accomplished by mining by conventional underground mining methods a relatively small amount, ordinarily in the range of 10 to 50 percent of the shale in the zone of the retort, to provide void space to accommodate the expansion of the shale that occurs on subsequent blasting to rubblize the shale to form passages through it. In the preferred in-situ retorting process, the rubblized shale is ignited at the upper end of the in-situ retort and air is passed downwardly through the shale to burn carbonaceous material in the shale. Hot combustion products from the combustion front in the shale pass downwardly through unretorted shale to heat that shale to a temperature at which the kerogen is converted to shale oil. The shale oil drains to the bottom of the retort and is delivered into suitable apparatus for pumping to the ground surface.
In-situ combustion processes for the recovery of oil from oil shale are described in U.S. Pat. No. 3,001,776 of Van Poollen; U.S. Pat. No. 3,661,423 of Garrett; U.S. Pat. No. 1,919,636 of Karrick and U.S. Pat. No. 2,481,051 of Uren. Van Poollen returns shale mined to provide the void space for rubblization to the top of the in-situ retort while combustion takes place in the retort. Karrick delivers the mined shale to an underground shaft in which it is retorted. Garrett retorts the mined shale at the surface, delivers the shale to a previously mined-out underground chamber for retorting in that chamber or builds an "underground retort" for retorting the mined shale.
One of the problems that confronts in-situ retorting of oil shale is the stability of the underground retort after the in-situ retorting operation has been completed. It has been reported that because of the loss of strength of the shale that occurs on retorting, the removal of kerogen from the shale in the in-situ retorting and the high compressive forces exerted by the shale in a retort that may be 700 feet or more high, shrinkage of 5 percent or more of the retorted rubblized shale may occur. The resulting void space at the top of the retort can cause subsidence of the overburden. Additionally, in most instances an in-situ retort will cross several water-bearing strata. It is necessary to seal the retort to prevent flow through the retort from one aquifer to another after the retorting has been completed.